Vak: See the ".get_mail_values" function, how it works with ".send_mail()", within odoo/addons/mail/wizard/mail_compose_message.py and tell me if that's what you're thinking. I imagine you'll want more examples, but by then I'll have had my coffee lol

I wanted to filter the `product_id` field on `mymodel.child2`
model, using `product_id` on `mymodel.child1` model on the same
parent.

Example:

The parent have 3 record on field `child1_ids`, each have
different product (product1, product2, product3), and I want the
product in field `child2_ids` can only select product1, product2,
or product3, not product4 or another product not set on
`child1_ids`. How I can do that?